Return loss is a measure of the amount of energy reflected back toward the RF source by a device. A high return loss (in dB) means that most of the signal energy gets into the device, or for a switch, most of the energy gets through the switch, if the switch itself has very little insertion loss. This is important for RF receiver front-ends where any loss, including loss of energy by reflections, directly impacts the gain and noise figure of the system.
The current HRL Laboratories' double-contact RF MEMS shown in FIG. 1 has a return loss that is less than 15 dB at 40 GHz when the switch is closed. This is too low for many switch networks where a return loss of greater than 20 dB is desired. An embodiment of the RF MEMS switch described herein is an improved double-contact RF MEMS that can achieve a return loss better than 30 dB with 3 dB or less degradation of isolation. This is an improvement of at least 15 dB in return loss over the current HRL Laboratories' practice.
Having a high return loss is important in any electrical system. HRL Laboratories' RF MEMS switch designs have been considered for use in a number of applications, including low-loss phase-shifters, system redundancy, millimeter wave beam switching, and tunable filters and oscillators. Improving the return loss, by increasing it, is desirable.
The prior art includes:                1. Loo, et. al., “Fabrication of Broadband Surface Micromachined Micro-electro-mechanical Switches for Microwave and Millimeter Wave Applications,” U.S. Pat. No. 6,331,257 of Dec. 18, 2001. This patent identifies the equivalent circuit of HRL Laboratories' switch as inductive in nature and that shunt capacitances could be used tas impedance matching circuits for the switch. FIG. 6 of this patent shows such a matching network using microstrip radial stubs. Microstrip radial stubs are elements well known for impedance matching circuits, but they are not necessary, and perhaps overly complicated, for a monolithic matching circuit.        2. Loo, et. al., “Monolithic Single Pole Double Throw RF MEMS Switch,” U.S. Pat. No. 6,440,767 of Aug. 27, 2002. The current practice of HRL Laboratories' RF MEMS double contact switches uses an elongate, moveable metal bar to connect the input and output transmission lines when the switch is closed. This metal bar has a width that is less than the width of the input and output transmission lines. The input and output transmission line width is nominally 50 ohms when the switch is used in a series microstrip configuration. Although some switches in the past have been fabricated with a bar the same width as the input and output transmission lines, the preferred practice is now to fabricate switches with a narrow connecting bar. This is because of fabrication yield and insertion loss reliability when the switch is closed. This type of switch is shown in the figures of that patent.        
In order to make the transition from the larger width line to the smaller width line, a short linear taper is used. The metal bar appears as a small inductor at frequencies where its length is much less than a wavelength. When the taper and metal bar are much less than a wavelength, the effect of the inductance is not noticeable and the return loss is very good. As the frequency increases, the inductance of the bar becomes significant, and the return loss degrades.
With respect to this technology, the inventors have taken into account the inductance of the metal bar, and have added integrated compensating capacitors to the electrode itself. These capacitors take the form of a widening or hump in the input and output lines close to the switch connection bar contacts in combination with the switch's ground plane. This results in a vast improvement in the return loss of the switch with the narrow metal connecting bar, especially at millimeter wave frequencies.
Aside from the patents listed above, documents related to other tapered structures related to monolithic circuits and switches are noted below which shows that most switch devices are capacitive in nature, thus requiring inductive matching such as tapered lines. Being inductive, HRL Laboratories' RF MEMS switch is apparently unique in the field of RF switches in that it requires a capacitive-type matching network.                1. Malherbe, A. G. Johannes and Steyn, Andre F., “The Compensation of Step Discontinuites in TEM-Mode Transmission Lines,” IEEE Trans. Microwave Theory Tech., Vol. MTT-26, No. 11, November 1978, pp. 883-885.—The use of short tapers between transmission line step discontinuities is a standard practice for microwave devices, such as diodes and FET's. In most cases, the input to the device has a parasitic capacitance, so narrowing the input transmission line adds some compensating inductance. Since the active part of the device is very small compared to a wavelength, linear tapers provide an acceptable input to and output from the device. This paper shows how to optimize this transition. This paper is listed to help give a physical basis to the current practice of RF line connection to microwave devices.        2. Jablonski, W., Jung, W., Gorska, M., Wrzesinska, H. and Zebrowski, Z. “Microwave Schottky Diode With Beam-Lead Contacts,” 13th International Conference on Microwaves, Radar and Wireless Communications. 2000, MIKON-2000, Vol. 2, pp. 678-681, 2000. And Maruhashi, Kenichi, Mizutani, Hiroshi, and Ohata, Keichi, “Design and Performance of a Ka-Band Monolithic Phase Shifter Utilizing Nonresonant FET Switches,” IEEE Trans. Microwave Theory Tech., Vol. 48, No. 8, August 2000, pp. 1313-1317.—Both of these papers have figures which show a linear taper from microstrip transmission line inputs and outputs into the device active region. These papers are cited as examples of current practice.        3. Rebeiz, Gabriel M. and Muldavin, Jeremy B., ‘RF MEMS Switches and Switch Circuits,” IEEE Microwave Magazine, December 2001, pp. 59-71.—This paper has a figure that shows that even for series RF MEMS, linear tapers are used to connect to the switch region.        